1. Field of the Invention
The invention relates to a material having antireflection, hydrophobic and abrasion resistance properties. The invention also relates to a process for depositing an antireflection, hydrophobic and abrasion resistant deposit on an organic or inorganic substrate.
2. Discussion of the Background
Organic or inorganic substrates, i.e. in particular plastics or vitreous substrates, coated with an antireflection film have numerous applications. Substrates prepared in this way are of particular interest in the following fields: high power lasers, solar, thermal and photovoltaic applications, integrated optical systems or in architectural applications, such as external vitreous panels. In connection with solar applications, plastic or vitreous substrates are used in optical systems for minimizing heat losses, for concentrating and focussing light energy and finally for protecting certain absorbent elements. Compared with vitreous substrates, plastic substrates are less expensive, more easily modelled, lighter and less fragile in the case of shocks.
Numerous practical solutions have been proposed for the deposition of thin antireflection layers on substrates, but very few of them are appropriate for the treatment of plastics materials.
GB 1 444 152, GB 1 530 833 and U.S. Pat. No. 3,996,067 describe conventional methods for the deposition of antireflection layers on various plastics, namely vacuum evaporation, reactive plasma and fluorination processes. However, these procedures have never undergone significant industrial development, because they are onerous, lead to local overheating and therefore deformations and consequently only permit coating to take place on one face at once.
Most of the proposed solutions require the heating of the antireflection film at temperatures of around a few hundred degrees and sometimes at between 1000.degree. and 1400.degree. C. (cf. particularly U.S. Pat. No. 4,286,024), which are well above those which can be withstood by a plastics material (max thermal stability 150.degree. C.).
U.S. Pat. No. 2,466,119 discloses a process for the preparation of reflecting films and/or antireflection multilayer products by hydrolysis and condensation of mixtures of titanium halides and/or silicon alkoxides. The porosity of these coatings is checked by varying the temperature. However, once again the obtaining of coatings having a good mechanical strength requires heating to temperatures well above those which can be withstood by conventional plastic.
U.S. Pat. No. 4,361,598 describes a process for the sol-gel deposition of a dense antireflection film based on SiO.sub.2 /TiO.sub.2 on stainless steel or silicon cells. In order to convert the polymer into dense oxide, it is necessary to perform a heat treatment at between 300.degree. and 600.degree. C. Therefore this process cannot be applied to plastics materials. Moreover, the refractive index range covered by this patent varies between 1.4 and 2.4, whereas for obtaining a monolayer antireflection coating on a plastic substrate, it is necessary to have a refractive index of approximately 1.22.
U.S. Pat. No. 4,397,666 describes a process for the deposition of a thick sol-gel film containing several components by acting on the viscosity by adding an agent increasing the latter. The thickness of the film obtained varies between 0.3 and 1.0 micron and these thicknesses are much too great compared with the thickness of 0.123 micron specified by Fresnel equations, when it is wished to make a monolayer antireflection deposit on a plastics material.
U.S. patent application Ser. No. 7,148,458 (NTIS) (corresponding to U.S. Pat. Nos. 4,929,278 and 4,966,812) describes a process for depositing antireflection films on plastic substrates consisting of synthesizing an ethanol gel in the SiO.sub.2 -B.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -BaO system until a certain molecular complexity is obtained, followed by the reliquefying of the said gel mechanically breaking certain interpolymer bridges. This leads to a porous film having a low refractive index (approx. 1.23) at ambient temperature, which permits an adaptation to plastic substrates. However, this film only has a mediocre abrasion resistance.
Moreover, although in the solar sector, plastics such as polycarbonates, polyacrylates, polyallyl carbonates and the like are of particular interest, vitreous substrates are also interesting, particularly in the field of general optics. However, it is clear that due to the approximately 4% reflection losses for each air-glass interface encountered (the average index of glass being 1.5), the loss balance for a complex optical system is often excessive.
Therefore opticians have long sought to create antireflection films by the use of physical processes such as vacuum sputtering or evaporation. However, these processes are often sophisticated and expensive and generally inappropriate for the production of large, cheap batches.
U.S. Pat. Nos. 2,348,704, 2,486,431 and 2,490,662 deal with the preparation of antireflection films on certain glasses by extracting the component which is most easily solubilizable under the action of appropriate leaching agents. However, although these processes make it possible to lower the refractive index, they are limited to glasses having a very specific composition. In the same way, U.S. Pat. Nos. 4,019,884 and 4,086,074 describe processes based on the capacity of certain vitreous compositions to develop a phase separation when subject to thermal corrosion. However, these processes can only be developed on even more specific vitreous compositions than those used in the preceding patents.
Other processes, more particularly described in U.S. Pat. Nos. 4,273,826, 4,446,171 and 4,535,026 lead to the formation of antireflection films having a limited abrasion resistance and which can therefore be very easily damaged by wiping.
Finally, U.S. Pat. Nos. 2,432,484 and 4,271,210 disclose the possibility of using alumina or silica colloids for producing antireflection dielectric coatings, making it possible to increase the porosity of said coatings and therefore lower their refractive indices. Although these processes have the advantage of being usable at low temperatures, the colloidal coatings obtained have a very poor mechanical strength and are in particular sensitive to any physical contact.